This invention relates to a method for the measurement of environmental stress cracking (E.S.C.) mainly occurring in plastic materials or metallic materials and to a device employed for practicing said method.
Whenever a material is put to actual use, it is always exposed to stress of some form or other. This stress at times is exerted directly on the material itself by some external factor. Sometimes it occurs internally in the material because some deforming force is applied thereto. Otherwise it is caused by the strain which remains in the material from the time of its fabrication. Most materials are used in some type of atmosphere (either gaseous or liquid) and, depending upon the kind of material, the kind of atmosphere and the magnitudes of the stress, some sustain cracks so seriously as to be rendered unserviceable in very short periods of time. This phenomenon is called "environmental stress cracking" or "environmental stress fracture" and is highly significance in the practical use of materials. A number of methods have been suggested for expressing the phenomenon in numerical terms. Broadly these are classified under two types: (1) methods wherein a prescribed deformation is imparted to the test piece in advance, the test piece is placed in an environmental liquid (or gas) at a fixed temperature and the time until the test piece sustains cracks is measured and (2) methods wherein a fixed magnitude of stress is imparted to the test piece prior to conducting the same measurement as above.
Among the methods of the former type is the well known ASTM D-1693-70 method developed by Bell Telephone Laboratory. Among those of the latter type, the Lander method (ASTM D-2552) is famous.
The Lander method comprises the steps of keeping a given test piece under a fixed magnitude of tension in an environmental liquid, detecting a fracture in the test piece and, at that precise moment, stopping a timer and automatically recording the time of said fracture.
The numerical value determined by this method is practically useful under actual conditions of use of a material. However, it is rarely exposed to a fixed magnitude of stress at all times. Since the device adopted for this method is unusually large and can test but a small number of test pieces at one time, the Lander method has not found widespread acceptance.
By contrast, the Bell Telephone method has been widely adopted as the standard method. This method, however, cannot be easily automated. Occurrence of cracks or fractures in the test piece is detected solely by visual evaluation conducted at fixed intervals by human beings. Thus, this method may give rise to large error in the numerical value to be obtained and at any rate requires much time and labor.
An object of this invention is to provide a method for the measurement of environmental stress cracking which is almost completely free from the influence of human error, is highly accurate and even permits automation.
Another object of this invention is to provide a device for the measurement of environmental stress cracking which is almost completely free from the influence of human error, is highly accurate and permits automation.